Adjustment of the frequency of a balance/balance-spring resonator can be achieved through modification of the inertia of the balance or of the elastic torque of the balance-spring.
It is known to use an index-assembly to modify the elastic torque of the balance-spring. An index-assembly generally includes two stop members intended to form the count-point, i.e. to define the length of the balance-spring strip, called the active length, which will work in contraction and expansion to provide the elastic torque of the resonator.
However, adjustment via an index-assembly is very sensitive, i.e. a slight shift of the stop members results in a large variation in frequency, and entails the development of micrometric screw systems designed to make adjustment more precise.